Can I keep my cigars in a tin-lined humidor?

A: Yes. I have seen this type of metal-lined humidor in antique shops (see example in photo). I’ve also seen some that look like they may have been designed more for cigarettes or pipe tobacco. But in your case, if you can control the temp and RH, it should work fine. You might want to keep your cigars in their cellos, though, to prevent any metallic taste getting into the wrappers.

Moreover, if it’s a nice looking box and in good condition, I would suggest lining the walls with Spanish cedar. Measure the base and walls and bevel cut the side panels as found in most cigar humidors.This way the wood can expand and contract. You could leave the underside of the lid as-is, or place a Spanish cedar panel over it. Make sure you get the appropriate size humidifier and a digital hygrometer. By going that route, it will impart that fresh cedar aroma which will help age your cigars nicely, too.